Charged secondary cell



United States Patent 3,347,707 CHARGED SECONDARY CELL James Southworth,Jr., Rocky River, and Robert E. Stark,

Avon Lake, (ihio, assignors to Union Carbide Corporation, a corporationof New York No Drawing. Continuation of application Ser. No.

336,043, Jan. 6, 1964. This application June 23,

1966, Ser. No. 560,012

5 Claims. (Cl. 136-28) This application is a continuation of theapplication of James Southworth, Jr. and Robert E. Stark, Ser. No.336,043, filed Jan. 6, 1964, now abandoned, for Charged Secondary Cellpursuant to Commissioners Notice of Feb. 11, 1966, 824 0.6. 1.

This invention relates to rechargeable electrolytic cells. Moreparticularly, this invention relates to rechargeable cells that areassembled in a charged state.

In conventional methods of manufacture for secondary cells the cellusually is assembled in a discharged state, cycled several times with anexcess of electrolyte present, the electrodes than are brought to thedesired state of charge either electrolytically or chemically,thereafter the electrolyte amount is adjusted, and the cell sealed. Itis readily apparent that the foregoing is a cumbersome andtime-consuming procedure.

Furthermore, the electrodes of prior art cells comprise a sinteredsubstrate holding an active mass which is introduced therein byimpregnation. Again an expensive and time consuming operation isnecessary in order to produce a satisfactory electrode.

It is the principal object of the present invention to simplify themanufacturing techniques of secondary cells and to provide acharge-bearing cell.

It is a further object to provide a secondary cell which requires noformation.

t is still another object to provide a hermeticallysealed secondary cellwhich employs pressed-powder electrodes.

Additional objects will readily present themselves to one skilled in theart upon reference to the ensuing specification and the claims.

The objects of this invention are achieved by a secondary cell whichcomprises, as assembled, a container, a means for hermetically sealingthe container, and chargebearing pressed powder electrodes containing anactive mass situated within the container. The electrodes are made froma powdered active mass pressed into an expanded metal grid, the powderedactive mass being substantially contained within the openings of themetal grid. Within the casing the electrodes are juxtaposed relative toeach other and have a tough, resilient separator which is resistant toelectrolyte deterioration interposed therebetween and in intimatecontact with the lateral faces thereof. An elecrolyte is present in thecell, but is held substantially immobilized, i.e., absorbed, Within thepressedpowder electrodes and within the separator. The positiveelectrode of the cell is always maintained under compression, thepressure being at least about 300 p.s.i. but not exceeding about 700p.s.i. A pressure in the range from about 400 to about 500 p.s.i. ispreferred.

Furthermore, the relative amounts of active mass within each electrodeare important as set forth in US. Pat. 2,571,927 issued on Oct. 16, 1951to G. Neumann et al. In the instant case the total charge-acceptingcapacity of the negative electrode must exceed that of the positiveelectrode by at least about percent and the charged portion of theuseful capacity of the negative electrode must be at least equal to thecharged portion of the useful capacity of the positive electrode. Also,the positive electrode must contain, in addition to a positively activemass, a predetermined amount of a cathodically-reducible mass asdisclosed in U.S. Pat. 2,934,581 issued on Apr. 26, 1960 to A. Dassler.

As used herein and in the appended claims the term useful capacity meansthe actual output measured in ampere hours or its equivalents of a givenelectrode in a secondary cell subjected to normal service. The foregoingterm encompasses both the charged and the uncharged portions of theelectrode. In the alternative, the useful capacity can be considered asthe product of the theoretical capacity and the efliciency of anelectrode.

As used herein and in the appended claims the term charge-acceptingcapacity means the capacity of the uncharged portion of the usefulcapacity of an electrode, expressed in ampere-hours or its equivalent.

The term active mass as employed herein and in the appended claims meansmaterial contained within an electrode and intended to enter into achemical and/or electrochemica-l reaction during some stage of theoperation of a cell.

The term oxide as used herein and in the appended claims is taken toencompass, in addition to binary oxygen compounds, also thecorresponding hydroxides, the corresponding oxyhydroxides or mixedoxides-hydroxides, and the corresponding hydrous oxides.

The reference to the valency state of the metal in any of the aboveoxides should be taken as indicating the average valency of the metal inthe oxide.

The terms charged electrode and charged cell as used herein and in theappended claims are taken to encompass 'both fully-charged andpartially-charged electrodes and cells.

The charged, pressed-powder electrodes are the crux of the instantsecondary cell. These electrodes employ an expanded metal grid havingopenings therein. Usually the grid comprises a plurality of metalstrands integrally joined at junctures therebetween; however, the gridcan also have a honeycomb structure or the like. The essentialrequirements of the grid are that it contain pockets or openings capableof receiving the powdered active mass which comprises the electrodes andthat it be capable of retaining the active mass substantially within theopenings once pressure is applied to the grid and the grid isplastically deformed. The grid itself must be an electrical conductorsince it serves as an internal current collector for the electrode.

Metal grids suitable for the purposes of the present invention can bemade by a proper stretching of a metal strip containing a plurality ofsubstantially unidirectional slits. The walls of the. grid which formopenings or pockets in the grid are preferably substantially normal tothe plane of grid.

Any metal can be employed as long as it is relatively inert with respectto the normal chemical and electrochemical reactions within the cell.For example, steel and nickel are commonly employed as the materials ofconstruction in a nickel-cadmium system. Since the conditions at thecadmium electrode are rimarily reducing, the selection of the gridmaterial is less critical than for the nickel-containing electrode. Atthe positive electrode, however, a more oxidation resistant material ispreferred. Typical of such materials for the nickel-containing electrodeare nickel or nickel-coated steel.

For a nickel-cadmium secondary cell the charged active mass of thepositive electrode is a nickel oxide having a relatively high oxygencontent. For a chemicallycharged cell the charged portion of thepositively-active mass is made up of a trivalent nickel oxide which canbe beta-nickelic oxide, gamma-nickelic oxide, delta-nickelic oxide andlambda-nickelic oxide. Preferably the deltanickelic oxide or thelambda-nickelic oxide are employed, and the remainder of the positiveactive mass is nickelous oxide. The trivalent nickel oxide can bepresent in the a positive electrode in an amount up to about 100 percentof the useful capacity of the cell. While it is preferred to have themaximum possible amount of trivalent nickel oxidepresent in the positiveelectrode, the amount canalso be as low as about 10 percent of theuseful capacity of the electrode and the remainder nickelous oxide.

The delta-nickelic oxide and the lambda-nickelic oxide are preferred forthe purposes of the present invention because of their relatively highoxygen content and stability.

The delta and lambda nickelic oxides can be represented by the formulaNi O -YH O. In the case of delta nickelic oxide x has a value from about2.8 to about 3.3 and Y has a value from about 1 to about 6. For lambdanickelic oxide x has a value from about 3.3 to about 3.9 and Y has avalue from about 1 to about 6. The delta-nickelic.

oxide is characterized by the X-ray diffraction pattern shown in Table Ibelow.

TABLE L-RELATIVE PEAK INTENSITIES OF X-RAY DIF- Table I presents therelative intensities and the position in angstroms of the peaks in theX-ray diffraction pattern of six samples of delta nickelic oxide ascompared to the X-ray diffraction pattern of the previously known betanickelic oxide. The beta nickelic oxide patterns are taken from Glemseret al., Z. Anorg. Chem., 261 (1950), pp. 26-42, and from AmericanSociety for Testing Materials, X-ray diffraction pattern card number6-0141. The relative intensity values in Table I are calculated on thebasis of the intensity of the strongest line in each pattern as beingequal to 100 and the other lines in proportion thereto. Table I clearlyillustrates the difference in the degree of crystal perfection whichdistinguishes the delta form of nickelic oxide from the beta form andindicates that these two crystal types are properly regarded asdifferent entities.

This form of the nickelic oxide can be preparedby oxidizingany nickelousoxide with an oxidizing agent such as an alkali metal hypochlorite, forexample. The only requirement is that the oxidation-reduction potentialof the oxidizing agent under the reaction conditions is higher than thatof the end product, i.e., the delta-nickelic oxide.

The lambda-nickelic oxide is characterized by the diffraction patternshown in Table II below;

oxide patterns are taken from American Society for Testing Materials,X-ray pattern card number 6-0075, and from a sample of the gammamaterial. Here again, it is apparent that the gamma and lambda crystaltypes are properly considered as different entities.

In comparison with this the delta crystal type is characterized by a2.35 line which is between, 25 and 40 percent of the 4.68 line and a1.41 line which is between and 17 percent of the 4.68 line. Suchdifferences in crystal perfection are considered significantcharacteristics in the identification of a crystal material.

The delta nickelic oxide is characterized as a poorly crystallinehexagonal system having an X-ray diffraction pattern characterized bythe reflections set forth in Table I and the characteristic ratios ofrelative intensities. In all cases the delta crystal type ischaracterized by having a much smaller ratio between the most prominentline and each of the minor lines than the beta form. For example boththe 2.35 and the 1.41 line are 80 percent of the 4.68 line for betanickelic oxide.

The lambda nickelic oxide is characterized as a poorly crystallinehexagonal system having an X-ray diffraction pattern characterized bythe reflections and intensities set forth in Table II. In comparing thegamma and lambda crystal patterns it is apparent that the gamma materialis characterized by a number of lines which do not appear in the lambdapattern. Moreover, the intensity of the lines that do appear in thelambda pattern show a substantially smaller ratio between the variouslines andthe strongest line in the pattern, i.e., the 7.11 line.

The particular X-ray technique and/or the instruments employed, thehumidity, the temperature, the orientation of the powder crystals andmany other variables, all of which are well known and understood tothose skilled in the art of X-ray crystallography can cause minorvariations in both the intensity and the position of the lines. Thesevariations even when relatively large pose no problem to the skilledX-ray crystallographer in establishing the identities of the crystaltypes involved. The X-ray data given herein to identify the variouscrystal types are not to exclude materials which fail to show all thelines or perhaps show a few extra ones that are permissible andconsistent with a poorly crystallized hexagonal system. Similarly,slight variations of the line position are within the acceptableparameters of the crystal structure.

The lambda-nickelic oxide can be prepared by precipitating, at roomtemperature or above, nickelous hydroxide from a nickel salt solution(for example, NiSO by sodium. hydroxide or potassium hydroxide,thereafter aging the resulting precipitate for about 24 hours or moreand at a pH in the range from about 9 to about 14, andv then oxidizingthe aged precipitate with an excess of sodium hypochlorite or the like.The product thus obtained is either a mixture of deltaandlambda-nickelic oxide or lambda-nickelic oxide alone, depending on theamount of sodium hypochlorite present. The larger the excess, the morelambda-nickelic oxide will be present. A more exhaustive description ofthe methods of preparation of deltaand lambda-nickelic oxides can befound TABLE II.-RELATIVE PEAK IN'IENSITIES OF X-RAY DIFFRACTION LINES INLAMBDA AND GAMMA RYSTAL TYPES OF NIC KELIC OXIDE Crystal Type 7.11 3. 542. 44 2. 39 2.19 2.12 1. 91 1. 79 1. 1.47 1. 41 1. 39 1.35 1. 32

Lambda 1 0 1i 1 2 2 Do 100 34 g g 3Q 2 1 ASTM Card 600075 gamma 100 8010 80 5 L 80 1O 80 10 10 (i0 60 10 Data from Glemser and Einerhand.

Nora-A line beneath the relative intensity indicates a broad peak.

Table II shows X-ray diffraction patterns of two samples of lambdanickelic oxide as compared to the X-ray diffraction pattern of gammanickelic oxide. The gamma nickelic in co-pending application Ser. No.336,025, filed of even date in the names of N. C. Cahoon and F. J.Krivanek having a common assignee.

In addition the positive electrode contains a cathodically-reduciblemass which serves to present hydrogen generation at the positiveelectrode in the event a polarityreversal is experienced within thecell, for example, in an instance where the cell is over-discharged. Thecathodically-reducible mass often referred to as anti-polar mass, doesnot disturb the function of the positive active mass during normalcharging and discharging of the cell. The cathodically-reducible masscan be present in the positive electrode in any amount; preferredamounts are those having a useful capacity in the range from about 10 toabout 60 percent of the useful capacity of the positive active mass. Theexact amount of the cathodicallyreducible mass present in the positiveelectrode can vary and is dependent primarily on the type of service towhich the cell will be subjected. In a nickel-cadmium secondary cellcadmium oxide or cadmium hydroxide normally is employed as theanti-polar mass.

If desired, the electrical conductivity of the positive electrode can beenhanced by the addition of a conductive powder such as graphite ormetallic nickel. The electrode can contain up to about 55 percent byweight, and generally about to 40 percent by weight, of conductivepowder. Similarly, short synthetic fibers can be admixed with theelectrode components so as to hold the powdered materials together.Typical fibers for this purpose range from 3 to 24 denier, about to 4;"long.

For optimum cell performance it is desirable to have an active materialsurface area as high as possible. Therefore the active constitutents arechosen in a finely-divided or powdered form. Generally the variousfinely-divided particles are admixed so as to form a substantiallyhomogeneous mixture which is then pressed or compacted into the metalgrid.

It has been found that upon addition of electrolytes and duringsubsequent charging the positive electrode tends to swell and force someof the active materials out of the metal grid. For this reason and inorder to assure long useful cell life the positive electrode must bephysi cally restrained as soon as the electrolyte is added andthereafter during the life of the cell. A holding pressure of at leastabout 300 psi. is necessary for this purpose. However, extremely highpressures have been found to interfere with the proper pore formation inthe positive electrode of the cell and thus are undesirable. Thepositive electrode should not be subjected to a holding pressureexceeding about 700 p.s.i.

In the negative electrode of the chemically-charged secondary cellemploying nickel and cadmium the negative active mass is made up ofcadmium metal and cadmium hydroxide which can be introduced into theelectrode initially either as cadmium oxide or hydroxide. The cadmiummetal is the chemically-charged portion of the active materials and ispresent in the negative electrode in an amount at least equal in usefulcapacity to the useful capacity of the trivalent nickel oxide present inthe corresponding positive electrode. It is preferred, however, that theamount of cadmium present in the negative electrode exceeds the amountof trivalent nickel oxide in the corresponding positive electrode by atleast about 5 percent in useful capacity. This excess of cadmium in thenegative electrode (the discharge reserve) is desirable in order todelay the expiration of the negative electrode during over-dischargeuntil the reduction of the anti-polar mass of the positive electrode iswell under way.

The uncharged active mass within the negative electrode i.e., thecadmium oxide or hydroxide, is always present in an amount having agreater charge-accepting capacity than the divalent nickel oxide presentin the corresponding positive electrode. In this manner an overchargereserve is provided within the negative electrode which prevents thenegative electrode from becoming fully charged and thus eliminates thepossibility of hydrogen evolution within the cell upon overcharging. Theexcess amounts of the charged and uncharged masses within the negativeelectrode are always. chosen so that the useful capacity of the negativeelectrode within a cell exceeds the useful capacity of the correspondingpositive electrode by at least about 10 percent.

If desired, conductive aids such as inert metal or graphite powders canalso be employed in the negative electrode.

. It is desired that the negative electrode in addition to a highsurface area also posseses high porosity for facili tating electrolyteaccess to the active materials during operation of the cell.

Various techniques can be employed to achieve this end. For example, forthe active mass a cadmium powder having large surface area (in the rangefrom about 1.6 to about 2.0 square meters per gram) is chosen. Cadmiummetal of this type can be obtained by the reduction of a reduciblecadmium compound such as cadmium oxide with aluminum powder in anaqueous alkaline solution. The resulting cadmium is in the form ofplate-like and acicular crystals and the metal powder mass remainsporous even after compacting because of particle shapeirregularities.

A method of preparation of such powdered cadmium metal can be found inUS. Ser. No. 335,798, filed of even date in the name of Stark and havinga common assignee, nOw Patent No. 3,297,433.

In an alternate method, in an instance where the cell utilizes analkaline electrolyte such as KOH, for example, solid alkali metalhydroxide can be admixed with the powdered active mass of the negativeelectrode and pressed into the expanded metal grid. Upon addition of theliquid electrolyte within an assembled cell the solid alkali metalhydroxide particles are dissolved and a porous negatively active mass isleft within the expanded metal grid.

In a further modification of the aforesaid method aluminum powder, aninert conductive powder, and cadmium oxide are admixed and compactedinto an expanded metal grid with or without an additional binder.Thereafter the aluminum powder is reacted with the cadmium oxide andwith alkali metal hydroxide in an aqueous bath of the alkali metalhydroxide to yield cadmium metal in a porous electrode. The pores areproduced by leaching out of the electrode the soluble aluminum reactionproducts. By suitably adjusting the relative amounts of the variouscomponents only part of the CdO in the admixture can be reduced to Cd,if desired, thus giving an electrode having an accurately-controlledratio of charged to uncharged active mass.

Another important constitutent of the secondary cell is the separatorbetween the electrodes. The separator must be tough, resilient, andresistant to electrolyte-degradation. In addition, the separator shouldbe capable of absorbing and retaining a substantial amount ofelectrolyte while still remaining gas permeable.

Separators found suitable for the present purposes are non-woven battsof felts made of synthetic fibers such as polyamide fibers,polypropylene fibers, or the like.

In general, alkaline electrolytes are contemplated for thehereindescribed secondary\ cells. Particularly preferred electrolytesare aqueous solutions of alkali metal hydroxides, such as aqueoussolutions of KOH and NaOH, for example. The concentration of thehydroxide within the electrolyte is not critical and is determinedusually by the type of cell and its intended application. Normally a 25to 35 percent KOH solution in water is employed.

The amount of electrolyte present within the cell is such that all of itis substantially held immobilized within the electrodes and theseparator. Again, the exact amount of electrolyte present is determinedby the type of cell and the contemplated use. An excess of electrolyte,however, will interfere with the ability of the cell to withstandovercharge. The proper amount in each application is readilydeterminable by the skilled artisan.

The chemically-charged secondary cell, the elements of which have beenfully described hereinabove is manupowdered mixtures containing activemasses in a finelydivided or powdered form are pressed into the openingsof the grid. This operation plastically deforms the grid so as to lockthe mixtures therein.

The positive electrode powder is packed into the grid usually so as toachieve about 70 percent of the theoretical density of the powderedmixture, i.e., about 30 percent porosity.v The porosity of the positiveelectrode will increase subsequently as it swells upon coming in contactwith the electrolyte and also upon cycling of the cell during use.

The mixture comprising the negative electrode is packed into therespectivegrid to a lesser extent-usually to about. 50 percent of thetheoretical density of the powder. The negative electrode is thenreacted to charge it to the desired degree as set forth above and thenwashed and dried.

The resulting pressed-powder electrodes are then trimmed to the desireddimensions, and contact strips are aflixed to the internal currentcollectors in any convenient manner. When a cell is assembled, apositive electrode is juxtaposed relative to a negative electrode and aseparator is placed inbetween the lateral faces of the electrodes and inintimate contact therewith. The entire assembly is held undercompression until it is inserted in a container, preferably atightly-fitting one. Once the electrolyte is added the applied pressureon the positive electrode is in the range from about 300 p.s.i. to about700 psi.

If it is desired to produce a cylindrical cell, alternate layers ofelectrodes and separators can be arranged in any suitable manner andthen wound spirally or coiled into a roll having the desired dimensions.The aforementioned compressive force can be applied conveniently duringthe winding operation, and the resulting roll inserted into a containerimmediately thereafter without a substantial release of the appliedpressure.

Oncethe electrode assembly is inserted into the container apredetermined amount of electrolyte is introduced therein wetting theelectrodes and the separator. The container then is hermetically sealedand the cell is 8 as a reducing agent for a part of the CdO. An exampleof a suitable formula for the negative mass is as follows:

Ingredients: Percent by weight Nickel powder 38.12 Aluminum powder 5.88Cadmium oxide 56.00

Nickel expanded metal is used as the electrode carrier grid. The carrieris expanded in continuous lengths from 0.004 or 0.005 inch thick stripmetal stock by convene tional commercial procedures. After expanding thecarrier stock is stretched to increase the overall thickn ss and enlargethe openings. The finished carrier grid for the negative electrode hasan overall thickness of 0.048 inch and a strand width of 0.029 inch.

The carrier grid is degreased to remove oil and dirt before furtherprocessing.

The negative electrode is made by passing in a continuous manner thenegative mass and the carrier grid in a vertical downward directionbetween the 14 inch diameter by 30 inch long horizontal rolls of aconventional two roll mill of the type used in powder metallurgy. Themetal carrier grid lays curved against the upper surface of one roll andmay be held under tension which is not great enough to deform it. Thenegative mass is metered into the roll gap alongside the metal carrierin such'a manner as to fill uniformly the multitude of small openings ofthe carrier.

The spacing between the mill rolls is adjusted to compact the negativemass in the carrier grid by reducing the overall grid thickness to 0.032inch. A packing of the. active mass of about 53% is obtained by thiscompacting. The reduction in grid thickness results frombending overvthe strands of the carrier which effectively locks the negative mass inplace.

The aluminum powder is caused to reduce a part of the CdO powder to Cdmetal by placing a coil of the anode stock interwound with a strip ofcorrugated nicket sheet, in the various environments as set forth inTable III. After the reduction reaction, the anode is washed and driedas shown in the table.

TABLE III.PROCESSING OF CHEMIOA'LLY CHARGED CADMIUM ANODE Step N 0.Time, Solution or Environment Solution Special Conditions HoursTemperature 4 N NaOH. 2 C. (il) Refrigerated N NaOH. 1 N NaOH. Heat tothis temperature. 4 A N NaOH. Do.

16 Demineralized Water 80-85 C Demineralized water is allowed to replacethe% N NaOH at the rate of about 1 Iiter/min./300 feet of anode. Waterin tank is ultrasonically vibrated and recirculated at about 10liters/min.

3 Acetone 2025 C 2 Vacuum 200 microns 200 C 1 Atmosphere- Nitrogen gas.Break vacuum with N ready for use. It should be noted that during theentire cell assembly operation the active materials are in a dry state.

As an example of the practice of the invention the following descriptionof a preferred method of manufacturing D size sealed high ratenickel/cadmium cells using charged pressed powder electrodes is given byway of embodiment. Other cell size may be made in essentially the samemanner with appropriate changes in dimensions of parts and components.

The negative mass consisting of essentially dry powders of nickel metal,aluminum metal and cadmium oxide are blended together (e.g. in atwin-shell blender) for 15 minutes. The nickel metal powder provideselectrode crush resistance and the aluminum powder serves later Nickelicoxide cathode (positive electrode) A positive mass consisting ofessentially dry powders of nickelic oxide, cadmium oxide, graphite, andpolyacrylic plastic fibers are mixed together in a mulling type mixerfor 15 minutes. The mix is then given a single pass through apulverizer. An example of a suitable formula is as follows:

The positive electrode is made by a rolling process in the same manneras the negative, except that the finished thickness after roll millingis 0.034" and the packing is 74 to 82%.

Finished electrode stock in widths up to 12 inches and coiled forhandling is slit into narrow width strips 1.90 inch wide and recoiled.

After slitting, the narrow width strips of electrode stock (bothpositive and negative) are cut to length. In this operation theelectrode is measured 01f taking account of variations in thicknessuntil a predetermined electrode volume has been obtained at which pointthe unit electrode is cut off. The unit positive electrodes are cut whena volume of 0.81 cubic inch has been reached. The unit negativeelectrodes are cut when a volume of .97 cubic inch has been reached. Ifthe nominal packing and electrode thickness values are maintained theunit positive electrode length will be 12.5 inches and the unit negativeelectrode length will be 16.0 inches. However, the thickness of bothpositive and negative electrodes may vary as much as $002 inch as afunction of the roll mill process. Cutting unit electrodes to apredetermined volume permits the use of any unit positive electrode withany unit negative electrode. Thus the proper balance in capacity betweenelectrodes is maintained in every finished cell.

Cut unit electrodes are next prepared for final cell assembly. Solidnickel current collector tabs 0.003 inch thick and 0.25 inch wide arespot welded to the prepared areas of each electrode. The finishedpositive electrode contains approximately 40 grams of positive masshaving a theoretical capacity of 8 ampere hours. The finished negativeelectrode contains 70 grams of negative mass having a theoreticalcapacity of 17.5 ampere hours.

One unit positive electrode and one unit negative electrode areassembled with separator strips as follows:

Non-woven nylon felt approximately 0.010 inch thick is used as theseparator material. It is first slit to a width of 2.17 inches then cutto length. Three pieces of separator are used for each cell. One pieceis cut several inches longer than the positive electrode and one pieceis cut several inches longer than the negative electrode. The thirdpiece is cut long enough to more than cover both sides of the negativeelectrode. A total length of 81 inches of separator strip is used ineach cell.

The electrodes and separator lengths are stacked with one piece ofseparator covering the inner side of the positive electrode and onepiece covering the mating inner side of the negative electrode. Thethird piece of separator covers both outer sides of the flat stackedassembly. Edges of the separator strips extend beyond the edges of theelectrodes at both top and bottom. The jelly roll winding operation isthe next process step and is accomplished by winding under pressure theelectrode-separator assembly into a coil between horizontal parallelrolls. Afterwinding, pressure is maintained on the jelly roll by thewinding rolls as the jelly roll is inserted into the cylindricalcontainer.

The cylindrical container consists of a drawn nickel plated steel canhaving a wall thickness of 0.010 inch. A thin (0.010 inch) disc of solidnylon resides in the bottom of the container under the jelly roll.

After winding, a 0.005 inch thick nickel current collector strip is spotwelded to the inside of the can near the open end and adjacent to thecluster current collector tabs of the negative electrode. This strip isthen spot welded to the group of tabs. The cell container then willbecome the negative terminal of the finished cell.

A vacuum is drawn on the partially completed cell and 14.5 cubiccentimeters of 25 percent potassium hydroxide solution is injected intothe cell.

The cell closure consists of six parts which include a 0.005 inch thicknickel current collector tab, a nylon gasket, a 0.035 inch thick nickelplated steel cover, a

laminated steel and rubber sealing disc, a coil spring, and a cap.

The sealing disc is seated over a small hole in the center of the cover.The coil spring is placed on top of the sealing disc. The cap is placedover the coil spring and spot welded to the cover. The coil spring andsealing disc form a resealable vent which will relieve excessive gaspressure which may develop within the cell on abuse.

The nickel current collector tab is spot welded to the bottom of thecover. The nylon seal is coated with asphalt. The cover assembly isinserted into the flanged rim of the nylon seal with the nickel currentcollector tab passing through a hole in the center of the nylon seal.

The nickel current collector tab on the cover assembly is spot welded tothe clustered current collector tabs on the positive electrode. Thecover assembly then becomes the positive terminal of the cell.

The cover assembly nylon seal combination is seated inside the open endof the cylindrical container and the open end of the container crimpedover the nylon seal. The cell is then passed through a draw die whichreduces the diameter of the container, thereby applying a radialcompression to the nylon seal between the container wall and the cover.

The charged cell is now finished and ready for use.

The finished charged cell illustrating a representative embodiment ofthe invention comprises an assembly of parts briefy described asfollows. A tightly wound jelly roll assembly of a central core, acharged positive pressed powder electrode strip, a charged negativepressed powder electrode strip, and several nylon mat separator stripspositioned between and around said electrode strips resides in acylindrical cell container. Extending upwards from the jelly rollassembly are two groups of metal tabs welded to the positive andnegative electrodes respectively. A negative connector strip is weldedto the negative tabs and to the inner wall of the container. A positiveconnector strip is welded to the positive tabs and to the underside ofthe metal top cover. The radially sealed top cover assembly comprises acupped nylon seal gasket with the positive connector strip passingtherethrough, a metal cover with small center hole positioned inside thenylon gasket, a metal cupped and flanged cap welded to the top of themetal cover and containing within the cup a metal coil spring bearing ona laminated steel and rubber sealing disc seated over the small hole inthe cover. The nylon mat separator and porous electrodes are understoodto be impregnated with the requisite volume of caustic electrolyte, anda thin inert, impervious insulating disk of sheet plastic resides in thebottom of the container.

The foregoing discussion has been directed primarily to nickel-cadmiumsecondary cells in the interests of clarity and convenience. It will bereadily apparent to the skilled artisan that the hereindisclosedadvances in the art can be applied equally well to other secondary cellsystems, for example, nickel oxide-zinc, manganese dioxide-cadmium,manganese dioxide-zinc, silver oxide-cadmium, silver oxide zinc, and thelike, without departing from the spirit and scope of this invention.

We claim:

1. A chemically-charged secondary cell which comprises, in combination,a container; a means for hermetically sealing said container; at leastone pressed-powder positive electrode comprising a mixture of a positiveactive powder, a cathodically reducible powder, and an inert conductivepowder compressed into a metal grid and a pressed-powder negativeelectrode comprising a mixture of a negative active powder with an inertconductive powder compressed into a metal grid, both positive andnegative electrodes situated within the container and juxtaposedrelative to each other; a tough, resilient separator resistant toelectrolyte deterioration interposed between the electrodes and inintimate contact with the lateral faces thereof; and an electrolytewhich is an aqueous solution of an alkali metal hydroxide substantiallyheld within the 11 electrodes and the separator; the positive activepowder consisting essentially of a stable nickel oxide having acomposition represented by the formula Ni O YH O.

wherein x has a value from about 2.8 to about 3.9 and wherein Y has avalue of from about 1 to about 6, and nickelous oxide, said stablenickel oxide and nickelous oxide being in relative amounts such thatfrom about to about 100 percent of the useful capacity of theelect-rodev is represented by said stable nickel oxide and the remainderby the nickelous oxide, the cathodically reducible powder beingfinely-divided cadmium hydroxide and the negative active powderconsisting essentially of cadmium and cadmium hydroxide; the cadmiumpowder within the negative electrode being present in an amount-suchthat the useful capacity of the cadmium powder at least equals that ofsaid stable nickel oxide present in the positive electrode and theamount of cadmium hydroxide. powder in the negative electrode being suchthat its charge accepting capacity exceeds that of the nickelous oxidepresent in the positive electrode by at least about 1-0 percent; and thepositive electrode being at all times under compression to theextent ofat least about 300 p.s.i., but not. exceeding about 700 p.s.i.

2. The cell in accordance with claim 1 wherein the cathodicallyreducible powder is present in the positive electrode in an amounthaving a useful capacity in the range from about 10 to about 60 percentof the useful capacity of the positive active mass.

3. The cell in accordance with claim 1 wherein the amount of cadmiumpowder present in the negative electrode exceeds the amount of saidstable nickel oxide in the positive electrode by at least about5 percentin useful capacity.

4. A chemically-charged secondary cell which comprises, in combination,a container; a means for hermetically sealing said container; at leastone pressed-powder positive electrode comprising an expanded metal gridhaving openings therein and a mixture of a positive active powder, acathodically reducible powder, and an inert conductive powder compressedand substantially contained within the openings of said metal grid and apressedpowder negative electrode comprising an expanded metal gridhaving openings therein and a mixture of a negative active powder withan inert conductive powder compressed and substantially contained withinthe openings of said metal grid, both positive and negative electrodessituated within the container and juxtaposed relative to each other; atough, resilient separator resistant to electrolyte deteriorationinterposed between the electrodes and in intimate contact with thelateral faces thereof; and an electrolyte which is an aqueous solutionof an alkali metal hydroxide substantially held within the electrodesand the separator; the positive active powder consisting essentially ofa stable nickel oxide having a composition represented by the formulawherein x has a value from about 2.8 to. about 3.9 and wherein Y has avalue of from about 1 to about 6, and nickelous oxide, said stablenickel oxide and nickelous oxide being in relative amounts such thatfrom about 10 to about 100 percent of the useful capacity of theelectrode is represented by said stable nickel oxide and the remainderby the nickelous oxide, the cathodically reducible powder beingfinely-divided cadmium hydroxide and being present in an amount having auseful capacity in the range from about 10 to about 60 percent of theuseful capacity of the positive active mass, the negative active powderconsisting essentially of cadmium and cadmium hydroxide; the cadmiumpowder within the negative electrodebeing present in an amount such thatthe useful capacity of the cadmium powder exceeds that of said stablenickel oxide present in the positive electrode by at least one-half ofthe useful capacity of the cadmium hydroxide present in the positiveelectrode and the amount of cadmium hydroxide powder in the negativeelectrode being such that its charge accepting capacity exceeds that ofthe nickelous oxide present in the positive electrode by at least about10 percent; and the positive electrode being at all times undercompression to the extent of at least about 300 p.s.i. but not exceedingabout 700 p.s.i.

5. The method of making a chemically-charged secondary cell whichcomprises the steps of:

(a) forming a chemically-charged positive electrode by pressing apowdered mixture comprising a positive active powder, a cathodicallyreducible powder and an inert conductive powder into the openings of ametal grid and in such manner as to lock the mixture within the grid,the positive active powder consisting essentially of a stable nickeloxide having a composition represented by the formula Nizo wherein x hasa value from about 2.8 to about 3.9 and wherein Y has a value of fromabout 1 to about 6, and nickelous oxide, said stable nickel oxide andnickelous oxide being in relative amounts such that from about 10 toabout percent of the useful capacity of the electrode is represented bysaid stable nickel oxide and the remainder by the nickelous oxide, thecathodically reducible powder being finelydivided cadmium hydroxide; (b)forming a chemically-charged negative electrode by pressing a mixturecomprising a negative active powder and an inert conductive powder intothe openings of a metal grid and in such manner as to lock the mixturewithin the grid, the negative active powder consisting essentially ofcadmiumand cadmium hydroxide, the cadmium powder being present in anamount such that the useful capacity of the cadmium powder at leastequals that of said stable nickel oxide present in the positiveelectrode and the amount of cadmium hydroxide powder being such that itscharge accepting capacity exceeds that of the nickelous oxide present-inthe'positive electrode by at least, about 10 percent;

(c) juxtaposing a resulting pressed-powder positive elec- ReferencesCited UNITED STATES PATENTS 2,131,592 9/1938 Lange et al. 13628 X2,934,581 4/ 1960 Dassler 136-9 3,031,517 4/1963 Peters 1366 3,075,033l/1963 Salkind 13624 WINSTON A. DOUGLAS, Primary Examiner.

A. SKAPARS, Assistant Examiner.

1. A CHEMICALLY-CHARGED SECONDARY CELL WHICH COMPRISES, IN COMBINATION,A CONTAINER; A MEANS FOR HERMETICALLY SEALING SAID CONTAINER; AT LEASTONE PRESSED-POWDER POSITIVE ELECTRODE COMPRISING A MIXTURE OF A POSITIVEACTIVE POWDER, A CATHODICALLY RECUCIBLE POWDER, AND AN INERT CONDUCTIVEPOWDER COMPRESSED INTO A METAL GRID AND A PRESSED-POWDER NEGATIVEELECTRODE COMPRISING A MIXTURE OF A NEGATIVE ACTIVE POWDER WITH AN INERTCONDUCTIVE POWDER COMPRESSED INTO A METAL GRID, BOTH POSITIVE AND ANEGATIVE ELECTRODES SITUATED WITHIN THE CONTAINER AND JUXTAPOSEDRELATIVE TO EACH OTHER; A TOUGH, RESILIENT SEPARATOR RESISTANT TOELECTROLYTE DETERIORATION INTERPOSED BETWEEN THE ELECTRODES AND ININTIMATE CONTACE WITH THE LATERAL FACES THEREOF; AND AN ELECTROLYTEWHICH IS AN AQUEOUS SOLUTION OF AN ALKALI METAL HYDROXID SUBSTANTIALLYHELD WITHIN THE ELECTRODES AND THE SEPARATOR; THE POSITIVE ACTIVE POWDERCONSISTING ESSENTIALLY OF A STABLE NICKEL OXIDE HAVING A COMPOSITIONREPRESENTED BY THE FORMULA